I have seen people post that they don’t bother gathering Thamium 9 - that they buy it, instead.
I have made 140 jumps so far in this playthrough. I have checked every space station. I have checked incoming traders. I have checked a on-planet trading stations. I have never seen Thamium 9 for sale.
Is there some special trick?
i can’t say if there is a trick but in can see many outposts with Thamium, i don’t remember if incoming traders have.
I am surprised you can’t see any at tradind stations.
I have seen it sold at planetary Trading Posts too.
Trick is to get economy scanner and jump only to affluent, opulent, flourishing, booming and prospering economies.
Thanks for the replies, folks. It must be something to do with my location - I’m still within 300 light years of my home planet.
Oh well. Looks like I’ll have to go to another part of the galaxy. Thamium sales don’t seem to be popular where I am.
Those economies are quite rare in all locations. For example within old hub there were only couple of discovered prosperous economies. And during travel between hubs I saw regions with no good economies at all. Scanners are highly recommended.
Definitely location. I couldn’t find any anyplace either, in the terminal or planet side. Then I jumped a blackhole and now it is everywhere 
Yes, that’s one of the main perks of building a trade terminal, lots of thamium
Trading post will have thamium, also incoming traders may have it. You dont have to go planetside to buy it, traders in freighter have the same goods as if you met them on planet surface.
Its space stations wher I have never seen thamium on sale.
Quantities are driven by the system economy wealth. Wealthier system have more on offer.
The problem seems to be, as I have found, and others have pointed out, that not all regions have Thamium for sale.
It may be necessary to go somewhere else in the galaxy.
Thats interesting, I never noticed that. When you say regions, do you mean galactic regions (think: Rento) or just clusters of planets that dont happen to have it?
It may not be a popular view, but it may actually bee quite interesting if there where whole regions devoid of some basic resources that would rely on some form of player cooperation or at least planning to be able to explore and survive.
It not being for sale at trading posts won’t mean you can’t just find it on planets.
As far as I’m aware there is not a single planet out there devoid of basic resources. (Although they are rare on a couple of em.)
Yes, I know the resources are actually spread quite evenly as minerals and the issue is just the trading. But then my imagination took off 
trade terminal on my freighter has thamium even when not offered on station or planet
does the offer change as you move around?
it’s always there, but if you buy thousands might take half hour for it to restock
Trading Posts or your freighter. The fastest way to find a trading post is to take a mission that involves delivering an item from the space station. Once at the trading post hit all the ships that land and buy their TH9 supplies.
To buy T9 at a trading post, you hit the post and all traders in the first 10 mins. Save game. Exit game. restart.
go buy again. do this about 10 times and you should be full.
Some place sell it at hundreds and others thousands. Base on economics.
Freighters have all that you need. Only thing is you will have to hop from ship to ship to get your needed quantities. Better than killing your feet walking around looking for Thamium when you need it to make warp cells. At least I think so.